Reduced superfluid density in a doped spin liquid candidate

A quantum spin liquid (QSL), which is an exotic magnetic state possibly carrying entangled and fractionalized excitation, would be an unprecedented stage for superconductivity. A promising candidate for a doped QSL is the organic triangular-lattice system, \k{appa}-(ET)4Hg2.89Br8, which hosts a non-Fermi liquid and magnetism of a QSL nature and shows superconductivity upon cooling. Here, we report that its superfluid density is anomalously reduced, indicating the existence of substantial incoherent spectral weight and weak superconducting phase stiffness. Moreover, the ratio of the superconducting transition temperature to the nominal Fermi temperature is as large as 0.1, meaning that electron pairing does not necessitate well-defined Fermi degeneracy, which the BCS scheme stands on. The observation of these features in a system free from competing orders, which complicate the physics in underdoped cuprates, offers a minimal ground to the longstanding issue of the missing superfluid density in doped Mott insulators.